. What chemical properties of water cause it to be a medium of life? — WZ, Pacific Palisades, CA
Water is such a remarkable chemical that I hardly know where to begin. First, it is one of the lightest, simplest molecules and yet it remains a liquid even at temperatures approaching 100° C, making it well suited as a medium for chemistry of all sorts. Second, it is an extremely good solvent for a vast range of ionic and organic materials, so that it is an ideal medium for the complicated chemical mixtures of biology. Third, water has enormous latent heats of melting and vaporization that make it hard to freeze and its evaporation very effective at cooling a hot animal.
. Which substance, calcium chloride or sodium chloride, melts ice faster and why? — MT, Fenton, MI
Without trying the experiment, I would expect sodium chloride to melt ice more quickly than calcium chloride simply because sodium chloride is more soluble in water. Anything that dissolves easily in water can melt ice, even sugar! A water-soluble material interferes with the crystalline structure of ice and, assisted by the tendency of everything to maximize randomness, converts the orderly arrangement of solid ice and soluble solid to the less orderly mixture of soluble material dissolved in liquid water. Both calcium chloride and sodium chloride are water soluble and thus melt ice, but sodium chloride is substantially more soluble than calcium chloride and ought to work faster.
However, molecule for molecule, calcium chloride will melt more ice than sodium chloride. That's because a single calcium chloride molecule decomposes into three separate ions in solution (one calcium ion and two chlorine ions). In contrast, a sodium chloride molecule only forms two separate ions in solution (one sodium ion and one chlorine ion). Since each ion contributes to the ice melting process, calcium chloride molecules are about 50% more effective than sodium chloride molecules. But even this increased molecular efficiency has a price: calcium ions are heavier than sodium ions, so a kilogram of sodium chloride actually yields more ions and more ice melting than a kilogram of calcium chloride. Still, salt is messy and corrosive so calcium chloride is often a good alternative.
. Could you give me the formula for figuring the wavelength of an ultrasound wave? — BH
The wavelength of any wave is equal to the speed of that wave divided by its frequency. In air, the speed of sound is about 330 meters per second, so an ultrasonic wave with a frequency of 50,000 cycles per second would have a wavelength of about 6.6 millimeters. Since sound travels much faster in liquids or solids, the wavelengths would be larger than in air.
. Does magnetism affect the growth of plants? If so, how? — JA, Somerville, MA
I am not aware of any effects of magnetism on plant growth. The effects of magnetism on most molecular processes are incredible slight and I don't see how any but the most extreme magnetic fields could affect plant growth.
. What is the physical nature of magnetism? Is it a wave or particle phenomenon or an undefined energy like gravity? — GA, Paisley, Scotland
Magnetism is one sector of the electromagnetic interactions of matter. From a classical perspective, magnetism consists of an energy-containing field that surrounds magnetic poles and that exerts forces on other magnetic poles. At a higher classical level, magnetism and magnetic fields are part of the full electromagnetic interaction, meaning that they are inextricably mixed with electricity and electric fields. Finally, from a full quantum mechanical perspective, magnetism is associated with energy-containing quantum fields, the fields of quantum electrodynamics, that govern the electric and magnetic interactions of matter. These quantum electrodynamic interactions are mediated by virtual photons, cousins of the real photons that include light and radio waves. From this quantum viewpoint, magnets interact with one another by exchanging virtual photons and, like all quantum objects, these photons are emitted and absorbed like particles but travel as waves. Thus magnetism is both a wave and particle phenomenon. It isn't undefined at all; in fact, quantum electrodynamics is probably the most well-established and precise theory in modern physics.
. How does a computer chip work? — JM, Austin, TX
A computer chip is also known as a digital integrated circuit. It is typically a thin wafer of silicon, cut from a single crystal of that element. The surface of the wafer has been chemically modified and it has had intricate patterns of aluminum wires and other structures cut and deposited photographically on its surface to form enormous numbers of transistors and other special structures. Each of these transistors is an electronically controllable switch. A tiny adjustment in the electric charge on the control element of one of these transistors—its gate—can dramatically alter that transistor's current carrying ability. These transistors work together to perform task that range from remembering one bit of information to multiplying two huge numbers together. The millions of transistors on a typical computer chip are able to perform extremely complicated tasks, as we see everyday in modern computers.
. Is there a device that would provide a variable output of radiated energy in the infrared that would be obtainable to experiment with? — NAT, Marion, SC
You can produce a broad range of infrared lights with a heat lamp. A heat lamp looks very dim because most of the thermal radiation it emits is in the infrared portion of the electromagnetic spectrum. Just attach the heat lamp to a normal light dimmer and you'll be able to vary its infrared output over a wide range of intensities. Its frequency range will also shift farther away from the visible as you lower its temperature by turning down the dimmer. If it produces more visible light than you want, you can put a filter in front of it that absorbs visible light while permitting infrared light to pass. Such filters are certainly available from filter companies such as Hoya or Corning but cheaper versions (perhaps even plastic filters) may be found through scientific supply companies.
. How does a roller coaster work?
A roller coaster is essentially a gravity-powered train. When the chain pulls the train up the first hill, it transfers an enormous amount of energy to that train. This energy initially takes the form of gravitational potential energy—energy stored in the gravitational force between the train and the earth. But once the train begins to descend the first hill, that gravitational potential energy becomes kinetic energy—the energy of motion. The roller coaster reaches maximum speed at the bottom of the first hill, when all of its gravitational potential energy has been converted to kinetic energy. It then rushes up the second hill, slowing down and converting some of its kinetic energy back into gravitational potential energy. This conversion of energy back and forth between the two forms continues, but energy is gradually lost to friction and air resistance so that the ride becomes less and less intense until finally it comes to a stop.
. What exactly are gravity waves and how are they measured? — AY, Wayne, PA
Gravity waves are deformations of space/time that propagate through space at the speed of light. While many motions of matter and energy are thought to emit gravity waves, those waves are normally extraordinarily weak. The only sources of detectable gravity waves are probably collapsing and colliding stars. Careful studies of the dynamics of binary star systems have shown that they also emit reasonably strong gravity waves, but those waves haven't been detected directly.
The two classes of gravity wave detectors currently in development or operation are large cryogenic bar detectors and laser interferometric detectors. A cryogenic bar detector tries to observe gravity waves by looking for vibrational excitations of huge metal bars. When a strong gravity wave passes through one of these bars, it should excite various vibrations in the bar that can be detected by sensitive motion sensors. A laser interferometric detector tries to observe gravity waves by looking at distance changes in the arms of a laser interferometer—a huge mirror system with laser beams bouncing back and forth within it. When a strong gravity wave passes through the mirror system, it should change the spacings of the mirrors enough to cause variations in the optical characteristics of the interferometer (for more info, see www.ligo.caltech.edu). So far, no gravity waves have been observed definitively.
. Why does the tower of Pisa lean? — CM, Edison, NJ
The tower was built long ago on unstable ground that was unsuitable for supporting such a tall and heavy masonry structure. For an object to remain upright indefinitely, its center of gravity must lie above its base of support and that base of support must be firm at all its edges. The tower's base of support had at least one edge that wasn't firm and that began to sink downward under the weight of the tower. Once this edge sunk a small distance, the tower's center of gravity shifted sideways so that it was above that weak portion of the base of support. This shift in the tower's center of gravity put even more stress on the weak part of the ground and caused additional sinking, additional tipping, and even more shifting of the tower's center of gravity. This process might have toppled the tower over by now were it not for recent efforts to stop the tipping. The base of the tower has been reinforced to prevent further tipping.